Optical resonators often include two or more mirror structures that define the resonator cavity. Optical resonators can be passive cavity devices as, for example, tunable Fabry-Perot (FP) filters. Active cavity devices also include a gain medium, such as a semiconductor or a solid-state material, inside the cavity between the mirror structures. A laser is an active cavity optical resonator.
The earliest and simplest reflective optical mirror structures were made by applying a thin metal coating to a substrate. Typical metals for the coating are: gold (Au), silver (Ag), and aluminum (Al). Typical substrates include glass and silicon, for example. The advantage of metal mirrors is optical reflectivity over a very broad spectral range. The disadvantages are that the reflectivity is sometimes not high enough for high performance applications and the metal coatings are soft and can be damaged.
For high performance cavities, requiring highly reflective and/or low absorption mirror structures, multi-layer thin dielectric film coatings are preferred. The refractive index differences associated with certain material systems such as ceramics enable the formation of 10 or more layers to achieve a reflectivity greater than 97%. When the substrate is an optical membrane, such as found in micro-optical electromechanical systems (MOEMS), the coatings can be one-half or more of the thickness of the membrane. A disadvantage of such coatings is that reflectivity spectrum might not be broad and/or consistent enough for some applications.
Different cavity designs have been used for FP filters. A flat-flat Fabry-Perot cavity supports a continuum of plane wave transverse modes. In a confocal Fabry-Perot cavity, where cavity length is equal to the mirror radius of curvature, all transverse modes are degenerate, i.e., all the transverse modes coexist on the same frequencies, or wavelengths, as the longitudinal mode frequencies or the longitudinal mode frequencies shifted by a half spectral period. A more typical configuration for a MOEMS tunable Fabry-Perot cavity is termed a hemispherical cavity. In such cavities, one of the reflectors is near planar and the other reflector is a curved or spherical reflector. The advantage of this configuration is reduced alignment criticalities over a confocal cavity because of the general radial homogeneity of the flat reflector and reduced parallelism criticalities over the flat-flat cavity. In such hemispherical configurations, spatial mode spectral degeneracy is not present.
One type of MOEMS Fabry-Perot tunable filter utilizes an electrostatically deflectable membrane. Such MOEMS membranes are coated to be reflective and then paired with a stationary or fixed spacer mirror device to form a tunable FP cavity/filter. Hemispherical cavities are then created by forming an optically curved surface on either the membrane or the fixed spacer mirror device, the other surface then being substantially flat. A voltage is applied between the membrane and an adjacent structure. The FP optical cavity's separation distance changes through electrostatic attraction as a function of the applied voltage.
There are a few main components that typically make up a MOEMS membrane device. In one example, the MOEMS membrane device includes a handle wafer support structure or gain structure, such as in the case of a vertical cavity surface emitting laser (VCSEL). An optical membrane layer is added to the handle wafer support or gain structure; and a deflectable membrane structure is then fabricated in this layer. This MOEMS membrane device includes an insulating layer separating the wafer support or gain structure from the membrane layer. This insulating layer is partially or completely etched away or otherwise removed to produce the suspended membrane structure in a release process. The insulating layer thickness defines an electrical cavity across which electrical fields are established that are used to electrostatically deflect the membrane structure.
One application for MOEMS membrane devices is as the tunable element(s) of the swept sources used in Optical Coherence Tomography (OCT). Optical coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of a sample. OCT is one example technology that is used to perform high-resolution cross sectional imaging. It is often applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from a sample or interfaces within the sample and a computer produces images of cross sections of the sample by using information on how the waves are changed upon reflection.
Fourier domain OCT (FD-OCT) currently offers the best performance for many applications. Moreover, of the Fourier domain approaches, swept-source OCT has distinct advantages over techniques such as spectrum-encoded OCT because it has the capability of balanced and polarization diversity detection. It has advantages as well for imaging in wavelength regions where inexpensive and fast detector arrays, which are typically required for spectrum-encoded FD-OCT, are not available.
In swept source OCT, the spectral components are not encoded by spatial separation, but they are encoded in time. The spectrum is either filtered or generated in successive frequency steps and reconstructed before Fourier-transformation. Using the frequency scanning swept source, the optical configuration becomes less complex but the critical performance characteristics now reside in the source and especially its frequency tuning speed and accuracy.
High speed frequency tuning for OCT swept sources is especially relevant to in vivo imaging where fast imaging reduces motion-induced artifacts and reduces the length of the patient procedure. It can also be used to improve resolution.
The swept sources for OCT systems have typically been tunable lasers. The advantages of tunable lasers include high spectral brightness and relatively simple optical designs. A tunable laser is constructed from a gain medium, such as a semiconductor optical amplifier (SOA) that is located within a resonant cavity, and a tunable frequency-selective element, such as a rotating grating, grating with a rotating mirror, or a Fabry-Perot tunable filter. Currently, some of the highest tuning speed lasers are based on the laser designs described in U.S. Pat. No. 7,415,049 B1, entitled Laser with Tilted Multi Spatial Mode Resonator Tuning Element, by D. Flanders, M. Kuznetsov and W. Atia, which is incorporated herein by the reference in its entirety. The use of MOEMS FP tunable filters combines the capability for wide spectral scan bands with the low mass, high mechanical resonant frequency deflectable MEMS membranes that have the capacity for high speed tuning. Specifically, in this design, the tunable laser uses a hemispherical cavity FP tunable filter that defines one end of the laser cavity.
Another field of application of tunable lasers, including those tuned with MOEMS movable membranes, is in spectroscopy. Tunable laser spectroscopy is used in many diverse areas, for example: gas detection and analysis, such as natural gas composition analysis; solid and liquid material identification and analysis, for example for different types of plastics, pharmaceuticals, or food products, to list a few examples.